1911 Encyclopædia Britannica/Castle-guard

CASTLE-GUARD, an arrangement under the feudal system, by which the duty of finding knights to guard royal castles was imposed on certain baronies, and divided among their knight’s fees. The greater barons provided for the guard of their castles by exacting a similar duty from their knights. In both cases the obligation was commuted very early for a fixed money payment, which, as “castle-guard rent” lasted on to modern times.

See J. H. Round, “Castle-Guard,” in Archaeological Journal, vol. lix., and “Castle-ward and Coinage,” in The Commune of London.  (J. H. R.)